A malignant neoplasm of the urinary bladder is one of the leading causes of cancer deaths in the United States. We have recently discovered that the placement of a silk suture in the bladder wall leads to development of atypical hyperplasia and tumorous growths of the transitional cells (Figs. 1, 2, 3, 4, 5). There is yet no evidence that these tumors are invasive. It is certain, however, that the urothelia of the urinary bladder have unique biological characteristic, namely, prominent atypical hyperplasia of urothelial cells develops in reaction to the foreign body in the bladder wall eventually resulting in formation of polypoid tumorous growths in one-third of the cases. We believe that the presence of foreign bodies in the bladder wall, if it is combined with administration of a subcarcinogenic dose of chemical carcinogen, will lead to the development of frankly malignant neoplasms, namely, bladder cancers. Our proposal, therefore, is to attest in the experimental animals the combined effects of foreign bodies and the administration of subcarcinogenic dose of N-methy-N-nitrosourea, a known bladder carcinogen. Cytological characteristics as well as invasiveness of the tumors will be examined by light and electron microscopy. In addition, we will explore whether or not that the effect of suture on the bladder mucosa is temporary and reversible. We believe that the foreign body in the bladder wall act as a promotor, therefore, its effect must be reversible. It is of critical importance to find out whether or not that foreign bodies in the bladder wall enhance development of bladder cancers.